The invention relates to thermoset epoxy polymers and, more specifically, to thermoset polymers derived from renewable resources.
Thermoset epoxy polymers are widely used in numerous industries, like coatings, adhesives, composites and electronic components encapsulation, because of their largely infusible and insoluble properties following curing. An example of particularly widespread use is as a coating of food cans to protect the food from contact with the reactive metal of the can. A cured epoxy system typically consists of a base epoxy resin and one or more additional components like hardener, catalyst, reactive flexible molecules, reactive diluents and a variety of inert additives. Many of these components are derived from petroleum and as such are not renewable. A very common moiety in these systems is bisphenol A, which is a widely used building block molecule in several plastic materials and epoxy resins. Bisphenol A has been known to have esterogenic properties since the 1930's. Unfortunately, the chemical bonds that link bisphenol A in polymer structures are not completely stable and the polymer may slowly decay with time, releasing small amounts of bisphenol A into materials with which it comes into contact, for example food or water. Recent studies have shown the widespread presence of tiny amounts of bisphenol A in many parts of the environment. Even at minute levels it may still exert estrogen-like effects on living organisms.
Alternative molecules that are derived from renewable sources and do not have estrogen-like properties would have obvious utility, particularly in such markets as food can coatings. They would also reduce demands for petroleum. Possible replacements for bisphenol-A and other epoxy linkers are the bisanhydrohexitols, stable cyclic ether diols derived by dehydration of sugar alcohols. These sugar alcohols are in turn simply produced by reducing hexose sugars derived from several bio-renewable resources such as corn syrup. An additional advantage that may apply to such materials is their biodegradation to harmless products. Notable anhydrosugars useful in this context include isosorbide, isomannide, and isoidide, although other derivatives not necessarily derived from hexose sugars may also be of value and fall within the scope of this invention.
It is known to make intermediates for epoxy resins from bisanhydrohexitols such as their glycidyl ethers. Examples of this art are to be found in U.S. Pat. No. 3,272,845 (Zech, et al.) and U.S. Pat. No. 3,041,300 (Morrison) which both describe methods of making the bisglycidyl ethers of isosorbide, isomannide, and isoidide, collectively referred to in the patents as isohexides.
Such epoxy intermediates are frequently water-soluble. In accordance with the present invention, we have shown that with suitable epoxy resin hardeners it is possible to make cold-cure epoxies in aqueous solution which gel and set hard and may be subsequently baked to give a final cure resin.